Various methods such as absorption, adsorption, membrane separation, and cryogenic separation are used to separate carbon dioxide from exhaust gas of chemical plants, power plants or large-sized boilers and from natural gas. An absorption or adsorption method is widely used when the concentration of exhausted carbon dioxide is low.
The method is widely used since it can be selectively separate. A particular gas that can be well absorbed or adsorbed into an absorbent or adsorbent; however, since the adsorbent or adsorbent is chemically altered during the separation, it is necessary to periodically replace the absorbent or adsorbent. On the other hand, an absorption method in which a liquid absorbent is used is widely used in purification of a large amount of exhaust gas or used in gas separation since it is easy to replace the absorbent and absorb a greater absorption capacity; however, the liquid absorbent may be chemically or thermally altered.
As carbon dioxide absorbents, amine solutions such as monoethanolamine (MEA), N-methyldiethanolamine (MEDA), diethanolamine (DEA), etc., are widely used. It is because, when reacting with carbon dioxide, an alkanolamine absorbent is chemically combined to thereby form carbamate compounds, and then, when heat is applied to the alkanolamine absorbent, the carbamate compounds are separated such that the carbon dioxide can be stripped and recovered and the alkanolamine absorbent can be regenerated. However, the process has some serious drawbacks in that: absorption capacity degradation may be caused by irreversible formation and decomposition of amine compounds due to impurities, such as sulfur dioxide (SO2), oxygen (O2), and nitrogen oxide (NOx), which are contained in a combustion exhaust gas, thereby causing corrosion of an absorption device; high thermal stability of carbamates formed by reaction with carbon dioxide requires a regeneration temperature to be 120° C. or higher, thereby causing excessive energy consumption (MEA requires 4.0 to 4.2 GJ per ton of carbon dioxide), excessive volatile loss of alkanolamine due to the high regeneration temperature (4 kg per ton in the case of using MEA), and replenishment of an absorbent; and carbon dioxide may be contaminated due to low vapor pressure of an absorbent during the regeneration process.
In order to resolve the drawbacks of the amine-based aqueous solvents, there have been reported various methods of physically absorbing carbon dioxide using organic solvents such as Selexol, IFPexol, NFM, etc. One important effect of the organic solvent absorbent is that a lower energy is required to recover carbon dioxide and recycle solvents since the absorption of carbon dioxide is achieved by a physical interaction between the solvent and carbon dioxide, not by the chemical bond as in the case of the aqueous amine absorbents. More specifically, in the case of using the amine absorbent, the recovery of carbon dioxide and the recycling of solvent require an energy-intensive, high-temperature stripping process; by contrast, in the case of the physical absorption. It is possible to recover carbon dioxide dissolved in the solvent by simply changing the pressure, not by increasing the temperature. However, the physical absorption method has some drawbacks in that in the case of separating carbon dioxide from a combustion gas having low pressure, the physical absorbents exhibit a carbon dioxide absorption capacity that is significantly lower than that of an aqueous amine solution, such that the circulation rate of the absorbent is high, thus requiring relatively large equipment. Accordingly, there is a need for the development of a novel chemical absorbent that has high thermal and chemical stability, and has low vapor pressure, so as to overcome the drawbacks of the general amine-based absorbents and organic solvent absorbents.
Recently, attempts have been made to reduce regeneration energy of chemical absorbents, including a method of using, as an absorbent, alkanolamine having steric hindrance around amine groups, and a typical example thereof is 2-amino-2-methyl-1-propanol (AMP) which is a primary amine. When reacting with carbon dioxide, AMP forms bicarbonate compounds that may be regenerated more readily than carbamates, thereby requiring 30% less regeneration energy compared to MEA; however, its CO2 absorption rate is less than 50% of the absorption rate of MEA.
As a method of increasing the absorption rate of AMP, Mitsubishi Heavy Industries, Ltd. and Kansai Electric Power Co., Inc. made a joint effort to develop a novel absorbent prepared by adding piperazine, which is a secondary cycloamine, to AMP (Japanese Patent No. 3197173). However, in the method, an excessive amount of piperazine is used such that precipitation occurs after absorbing carbon dioxide, and when piperazine is reacted with carbon dioxide, stable carbamate compounds are formed in addition to bicarbonates, such that a regeneration process is difficult to perform.
Further, there is also known a method of using, as a CO2 absorbent, alkali carbonate, such as sodium carbonate or potassium carbonate, instead of using a primary alkanolamine absorbent; however, the method has a problem of low CO2 absorption rate. As a method of increasing a CO2 absorption rate, WO2004-089512 A1 discloses a method of adding piperazine or its derivative to potassium carbonate in which a CO2 absorption rate of potassium carbonate is significantly increased by; however, the method also has a drawback in that precipitation occurs when using potassium carbonate.